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United States Patent |
6,048,413
|
Park
,   et al.
|
April 11, 2000
|
Duplex stainless steel with high corrosion resistance
Abstract
A corrosion resistant duplex stainless steel having an austenite-ferrite
duplex phase matrix, less content of the expensive nickel and higher the
resistance to both stress corrosion cracking and pitting in environments
containing chloride ion is disclosed. The stainless steel is also scarcely
influenced by the aging heat treatment. This stainless steel includes
20-30 wt % chromium, 3-9 wt % nickel, 3-8 wt % molybdenum, 0.20 wt % or
less carbon, 0.5-2.0% silicon, 3.5 wt % or less manganese, 0.2-0.5%
nitrogen and a balance of iron. The stainless steel may include at least
one element selected from the group of 1.5 wt % or less titanium, 3 wt %
or less tungsten, 2 wt % or less copper, and 2 wt % or less vanadium and
include at least one element selected from the group of 0.001-0.01 wt %
boron, 0.001-0.1 wt % magnesium, 0.001-0.1 wt % calcium, and 0.001-0.2 wt
% aluminum.
Inventors:
|
Park; Yong Soo (Joongang Heights Villa 532, 1000-3, Bangbae-Dong, Seocho-Ku, Seoul, KR);
Kim; Young Sik (Hyundai Apt. 205-901, 600, Yongsang-Dong, Andong, Kyungsangbook-do, KR)
|
Appl. No.:
|
819176 |
Filed:
|
April 28, 1997 |
Foreign Application Priority Data
Current U.S. Class: |
148/325; 148/327; 420/67; 420/68 |
Intern'l Class: |
C22C 038/44 |
Field of Search: |
148/325,327
420/67,68,69
|
References Cited
U.S. Patent Documents
4915752 | Apr., 1990 | Culling | 148/325.
|
5238508 | Aug., 1993 | Yoshitake et al. | 148/325.
|
5298093 | Mar., 1994 | Okamoto | 148/325.
|
Foreign Patent Documents |
0 757 112 A1 | Feb., 1997 | EP.
| |
Primary Examiner: Yee; Deborah
Attorney, Agent or Firm: Townsend and Townsend and Crew LLP
Parent Case Text
This is a Continuation of application Ser. No. 08/444,388, filed May 18,
1995 now abandoned, the disclosure of which is incorporated by reference.
Claims
What is claimed is:
1. A corrosion resistant duplex phase stainless steel consisting
essentially of:
20-30 wt % chromium, 3-8.5 wt % nickel, 5.1-8 wt % molybdenum, 0.20 wt % or
less carbon, 0.5-2.0 wt % silicon, 3.5 wt % or less manganese, 0.25-0.5 wt
% nitrogen and the balance iron.
2. The stainless steel according to claim 1, further comprising:
at least one element selected from the group of 1.5 wt % or less titanium,
3 wt % or less tungsten, 2 wt % or less copper, and 2 wt % or less
vanadium.
3. The stainless steel according to claim 1, further comprising:
at least one element selected from the group of 0.001-0.01 wt % boron,
0.001-0.1 wt % magnesium, 0.001-0.1 wt % calcium, and 0.001-0.2 wt %
aluminum.
4. The stainless steel according to claim 2, further comprising:
at least one element selected from the group of 0.001-0.01 wt % boron,
0.001-0.1 wt % magnesium, 0.001-0.1 wt % calcium, and 0.001-0.2 wt %
aluminum.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to duplex phase stainless steels
having austenite-ferrite duplex phase matrix and good resistance to both
stress corrosion cracking and pitting, and suitable for use in the areas
of heat exchangers using seawater as cooling water, tanks and pipes of
desalination plants, FGD (Flue Gas Desulfurization) equipments fossil
power plants, tubes and pipes of refineries and petrochemical plants,
equipments of chemical plants and waste water disposal plants.
2. Description of the Prior art
It has been typically noted that stainless steels are special steels having
excellent corrosion resistance in comparison with the other alloy steels.
However, typical commercial stainless steels have no good resistance
against both stress corrosion cracking (SCC) and crevice corrosion, such
as pitting, so that the typical stainless steels can not be used as
materials of equipments for the environments including high concentration
of chloride ion. In this regard, titanium alloy or nickel-based super
alloy instead of the typical stainless steels are used as the material of
equipments for the environments including high concentration of chloride
ion.
However, the titanium alloy and the nickel-based super alloy are not only
limited in their production amounts but also very expensive in comparison
with the typical stainless steels. In this regard, there have been
continuous studies on the development of improved corrosion resistant
stainless steel by controlling composition of alloy elements of the
stainless steel.
For example, both AISI 316 (Sammi Specialty Steel Co. Ltd., Korea) produced
by addition of 2-3% of Mo to austenitic stainless steel of AISI 304 and
the austenitic stainless steel such as nitrogen-laden AISI 317 LNM
(Creusot-Loire Industrie, France) being noted to have somewhat improved
the corrosion resistance of the stainless steel. However, those stainless
steels are also noted to have poor resistance against SCC in specified
corrosion environments, such as chloride ion-containing solution under
tensile stress. In an effort to overcome the problems of those stainless
steels, duplex phase stainless steel having austenite-ferrite duplex phase
matrix has been proposed.
However, the corrosion resistance of the duplex phase stainless steel will
be reduced in the case of aging heat treatment of the stainless steel. In
this regard, the corrosion resistance of the stainless steel goods can not
help being reduced when the steel is heated such as by welding. Such
reduction of corrosion resistance of the typical corrosion resistant
stainless steel due to the aging heat treatment is caused by
transformation of the ferrite phase of the duplex phase stainless steel
into austenite II phase and sigma phase including large amount of chromium
and molybdenum and having high hardness.
U.S. Pat. No. 4,500,351 discloses a cast duplex phase stainless steel which
generates no pitting in anode polarization at temperatures of 50.degree.
C.-78.degree. C. in 1 mole NaCl solution but generates crevice corrosion
at 47.5.degree. C. in 10% FeCl.sub.3.6H.sub.2 O.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a corrosion
resistant duplex phase stainless steel which has an austenite-ferrite
duplex phase matrix, and which has reduced content of the expensive nickel
and improved resistance to both stress corrosion cracking and pitting in
chloride ion-containing environment.
It is another object of the present invention to provide a corrosion
resistant duplex phase stainless steel which is scarcely influenced by the
aging heat treatment but has improved resistance to both stress corrosion
cracking and pitting.
In order to accomplish the above object, the present invention provides a
corrosion resistant duplex phase stainless steel comprising 20-30 wt %
chromium, 3-9 wt % nickel, 3-8 wt % molybdenum, 0.20 wt % or less carbon,
0.5-2.0% silicon, 3.5 wt % or less manganese, 0.2-0.5% nitrogen and a
balance of iron.
The stainless steel may include at least one element selected from the
group of 1.5 wt % or less titanium, 3 wt % or less tungsten, 2 wt % or
less copper, and 2 wt % or less vanadium.
The stainless steel may include at least one element selected from the
group of 0.001-0.01 wt % boron, 0.001-0.1 wt % magnesium, 0.001-0.1 wt %
calcium, and 0.001-0.2 wt % aluminum.
BRIEF DESCRIPTION OF THE DRAWING
The above and other objects, features and other advantages of the present
invention will be more clearly understood from the following detailed
description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a graph showing the results of stress corrosion cracking test of
alloy samples of this invention in a boiling solution of 42% MgCl.sub.2 in
accordance with variation of ferrite contents of the alloy samples;
FIGS. 2A and 2B are graphs comparatively showing the results of stress
corrosion cracking test of the alloy samples (FIG. 2A: samples 7, 8 and 9;
FIG. 2B: samples 10, 11 and 12) of this invention and AISI 304 stainless
steel in the boiling solution of 42% MgCl.sub.2 ;
FIG. 3 is a graph comparatively showing the results of pitting test
(immersion test) of the alloy samples of this invention (sample Nos. 1, 2,
3, 4, 5 and 6), AISI 316L stainless steel and SUS M329 stainless steel;
FIG. 4 is a graph comparatively showing the results of pitting test (anodic
polarization test) of the alloy samples of this invention (sample Nos. 1,
2, 3, 4, 5 and 6), AISI 316L stainless steel and SUS M329 stainless steel;
FIG. 5 is a graph comparatively showing the results of pitting test (anodic
polarization test: 70.degree. C., 0.5N HCl+1N NaCl) of the alloy samples
of this invention (sample Nos. 31, 32, 33, 34, 35, 36 and 37) and SAF 2507
stainless steel;
FIG. 6 is a graph comparatively showing the results of pitting test (anodic
polarization test: 80.degree. C., 22% NaCl) of the alloy samples of this
invention (sample Nos. 31, 32, 33, 34, 35, 36 and 37), AISI 316L stainless
steel (Sammi Special Steel Co. Ltd., Korea), SAF 2507 stainless steel
(Sandvik Steel Co., Sweden), Zeron 100 stainless steel (Weir Co., U.K.)
and UR52N+ stainless steel (Creusot-Loire Industrie Co., France);
FIGS. 7A and 7B are graphs showing the results of pitting test (anodic
polarization test: 50.degree. C., 0.5N HCl+1N NaCl) of alloy samples 31
and 37 of this invention in accordance with aging heat treatments
respectively; and
FIG. 8 is a graph showing the results of pitting test (anodic polarization
test: 50.degree. C., 0.5N HCl+1N NaCl) of UR52N+ stainless steel
(Creusot-Loire Industrie Co., France) in accordance with aging heat
treatments.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The duplex phase stainless steel of the present invention includes 20-30 wt
% chromium, 3-9 wt % nickel, 3-8 wt % molybdenum, 0.20 wt % or less
carbon, 0.5-2.0% silicon, 3.5 wt % or less manganese, 0.2-0.5% nitrogen
and a balance of iron.
In order to not only reduce the influence of aging heat treatment but also
improve the corrosion resistance of the resulting stainless steel, further
the stainless steel may be added with at least one element selected from
the group of 1.5 wt % or less titanium, 3 wt % or less tungsten, 2 wt % or
less copper, and 2 wt % or less vanadium.
In order to improve hot workability, the stainless steel may be added with
at least one element selected from the group of 0.001-0.01 wt % boron,
0.001-0.1 wt % magnesium, 0.001-0.1 wt % calcium, and 0.001-0.2 wt %
aluminum.
When comparing the instant corrosion resistant duplex phase stainless steel
with the typical stainless steels, the instant stainless steel has a
relatively higher critical pitting temperature of about 95-90.degree. C.
in 10% FeCl.sub.3.6H.sub.2 O solution. In addition, the instant stainless
steel not only has a high passive region not less than 1000 mV but also
scarcely generates pitting in an anodic polarization, thus to have
improved corrosion resistance and to substitute for expensive titanium
alloy or expensive nickel-based super alloy.
The instant stainless steel has shown scarcely increase in the corrosion
rate after aging heat treatment so that the stainless steel has an
advantage that it is scarcely influenced by the aging heat treatment. The
reason why the instant stainless steel is scarcely influenced by the aging
heat treatment is judged to be resulted from appropriate control of
austenite-ferrite phase ratio. In the case of addition of titanium to the
stainless steel, titanium compound is formed in the steel as a result of
the aging heat treatment and the titanium compound retards transformation
of ferrite into sigma+austenite II. Such retardation of transformation is
also judged to cause the instant stainless steel to be scarcely influenced
by the aging heat treatment.
In the present invention, the stainless steel has the highest corrosion
resistance when its ferrite content is about 40-50 wt %. The reason why
the stainless steel has the highest corrosion resistance in the case of
the ferrite content of about 40-50 wt % is that the mechanically hard
ferrite phase under low or middle stress acts as an obstacle in inducing
slip. The ferrite phase also electrochemically acts as the anode for the
austenite phase in the chloride environment so that the austenite phase
becomes the cathode. Such an austenite phase retards cracking during
dissolution of ferrite phase. In a given plastic model, the austenite
phase has a stress component smaller than that of the ferrite phase but
has a high thermal expansion coefficient at a high temperature so that the
austenite phase is more easily shrunk than the ferrite phase in the case
of cooling. In this regard, a compressive residual stress is generated in
the outside of the interface between the phases and limits possible
cracking so that the phases in the matrix limit cracking propagation.
Therefore, the ferrite of about 50 wt % results in the highest corrosion
resistance of the stainless steel.
The elements of the duplex phase stainless steel of this invention have
their intrinsic functions and are preferably limited in their contents due
to the following reasons.
Chromium
Chromium (Cr) is an element for ferrite stabilization and acts as one of
important elements for corrosion resistance of the resulting alloy. In
order to form the austenite-ferrite duplex phase matrix in the alloy
(stainless steel) of this invention, at least 20 wt % chromium should be
included in the alloy in consideration of balance of carbon, nitrogen,
nickel, molybdenum, silicon and manganese. However, when considering the
phase ratio of the austenite-ferrite duplex phases, mechanical
characteristic and cost of resulting stainless steel, it is not preferred
to add 30 wt % or more chromium to the alloy.
Nickel
Nickel (Ni) is a strong element for austenite stabilization and a
profitable element for corrosion resistance of the resulting alloy so that
at least 3 wt % nickel is preferably included in the alloy. In order to
not only keep the desired phase ratio of the austenite-ferrite in
accordance with balance of the other elements but also reduce the cost of
the resulting alloy, the content of nickel is limited to 9 wt % and more
preferably ranged from 4 to 8 wt %.
Molybdenum
Molybdenum (Mo) is an element for ferrite stabilization and acts as one of
important elements for corrosion resistance of the resulting alloy. It is
preferred to limit the content of molybdenum to 8 wt % in view of
workability and phase stability during heat treatment. More preferably,
the content of molybdenum is ranged from 4.5 to 7 wt %.
Carbon
Carbon (C) is one of important elements for mechanical variable as it is a
strong element for austenite stabilization. However, as the carbon will
reduce both corrosion resistance and hot workability, it is preferred to
limit the content of carbon up to 0.20 wt %. It is more preferable to
limit the content of carbon up to 0.03 wt % in view of corrosion
resistance of the resulting alloy.
Silicon
Silicon (Si) is an element for ferrite stabilization and gives a
deoxidation effect during the melting and acts as an element for improving
oxidation resistance of the resulting alloy. However, excessive silicon
will reduce both toughness and ductility of the resulting alloy so that
the content of silicon is preferably ranged from 0.5 to 2.0 wt %. In
addition, it is also preferred to limit the content of silicon up to 1.0
wt % in view of corrosion resistance of the resulting alloy.
Nitrogen
Nitrogen (N) is a strong element for austenite stabilization and acts as
one of important elements for corrosion resistance of the resulting alloy.
When the nitrogen is included along with molybdenum in the alloy, the
effect of nitrogen is more enhanced due to improvement of passive layer
characteristic. When reducing the content of carbon in the resulting alloy
in order for improving the intergranular corrosion resistance, it is
possible to compensate for reduced mechanical performance of the alloy by
addition of nitrogen. The content of nitrogen is preferably limited up to
0.5 wt % in view of both balance of the other elements and desired phase
ratio of austenite-ferrite. In addition, it is also preferred to let the
content of nitrogen not less than 0.15 wt % in view of corrosion
resistance of the resulting alloy.
Copper
Copper is an element for austenite stabilization and strengthens the matrix
of the resulting alloy and increases the strength of the resulting alloy.
However, excessive copper will reduce corrosion resistance of the
resulting alloy. In sulfuric acids, Cu increases corrosion resistance. It
is prefered to have Cu under 2 wt %.
Titanium
Titanium is an element having deoxidation effect during the melting and may
be added to the alloy in order for improving the intergranular corrosion
resistance. When adding the titanium for resistance against intergranular
corrosion, it is required to consider relation of the titanium with the
amount of added carbon. The content of Ti is preferably ranged from 0.5 to
1.5 wt % to increase the corrosion resistance in environments containing
chloride after the aging heat treatment.
Each alloy sample of the present invention is produced as follows.
After making prediction about intended ferrite content by calculating both
chromium equivalent and nickel equivalent considering influence of
nitrogen, the gradients of commercially pure grade electrolytic iron
(99.9% purity), chromium (99.6% purity), molybdenum (99.8% purity), nickel
(99.9% purity), Fe--Si and Fe--Cr-N are melted in a magnesia crucible of a
high frequency induction furnace under gaseous nitrogen ambient and,
thereafter, formed into an ingot using a sufficiently preheated metal mold
or sand mold.
The chromium equivalent (Cr.sub.eq) and the nickel equivalent (Ni.sub.eq)
are calculated by the following equations 1 and 2 respectively.
Cr.sub.eq =%Cr+1.5%Si+% Mo+% Cb-4.99 (1)
Ni.sub.eq =%Ni+30%C+0.5%Mn+26(% N-0.02)+2.77 (2)
For the production of wrought material, the ingot is machined into an
appropriate size by machining or grinding and, thereafter, subjected to
soaking at a temperature of 1050-1250.degree. C. and for a soaking time of
at least 1 hr/inch. After the soaking, the ingot is subjected to the hot
rolling and cooled in water. As there may be a chance of cracking in the
hot plate due to precipitation of sigma phase in the case of lower
finishing temperature of the hot rolling, the finishing temperature of the
hot rolling should be kept at at least 1000.degree. C. In order to remove
oxides formed on the hot plate as a result of the hot rolling, the ingot
is rolled to 1-2 mm thickness through cold rolling after pickling in a
solution of 10% HNO.sub.3 +3% HF at a temperature of 66.degree. C.
In order to let castings, hot-rolled products or cold-rolled products of
the stainless steel of the invention have optimal performance, it is
preferred to subject the products to annealing for 1-2 min/mm (thickness)
at temperature of 1100-1150.degree. C. in accordance with compositions of
alloy. After the annealing, the products are again subjected to pickling
in a solution of 10% HNO.sub.3 +3% HF at temperature of 66.degree. C. so
as to remove oxide scales from the products.
Test for the stress corrosion cracking (SCC) resistance of the instant
stainless steel was carried out by the SCC test of constant extension rate
test proposed by standard G 36-75 of ASTM (American Society for Testing
and Materials). That is, the resulting alloy samples of the invention were
immersed in a corrosion cell containing 42% MgCl.sub.2 at a constant
temperature of 154.degree. C. and the fracture times of the samples in the
corrosion cell were measured. In this case, the longer fracture time of an
alloy sample, the higher SCC resistance the alloy sample has.
The resistance against pitting corrosion of the alloy samples of this
invention was measured by both weight loss test and anodic polarization
test.
The weight loss test for the instant alloy samples was carried out through
a method proposed by ASTM G48 or its adherent method. For example, the
pitting corrosion rate of the alloy samples was measured from the weight
loss rate of the samples by immersing the samples in a solution of 10 wt %
FeCl.sub.3.6H.sub.2 O for 24 hours at a constant temperature of 50.degree.
C. In the weight loss test, the less weight loss of an alloy sample, the
higher pitting corrosion resistance the alloy sample has.
In the anodic polarization test of the alloy samples for testing the
pitting corrosion, 0.5N HCl+1N NaCl solution or 22% NaCl solution was used
as the test solution. A potential-current curve was obtained while
scanning, using potentiostat, the potential from corrosion potential to
more anodic potential and, thereafter, the pitting corrosion resistance of
the alloy was measured from the critical current density, passive current
density and pitting potential. The pitting corrosion resistance of the
alloy is in inverse proportion to both the critical current density and
the passive current density. In addition, the pitting corrosion resistance
is in proportion to the pitting potential and this means that the pitting
corrosion resistance is increased when the curve moves leftward.
A better understanding of the present invention may be obtained by looking
at the following examples which are set forth to illustrate, and are not
to be construed to limit, the present invention.
EXAMPLE I
With substance of electrolytic iron, chromium, nickel, molybdenum, Fe--Si,
Fe--Cr--N, all commercially adoptable quality grade, 12 kg of each of
alloy specimens was prepared according to the compositions as indicated in
Table 1, under a nitrogen environments in a high frequency induction
furnace. At the moment parts which contains pores were detected by
radiographic method, and were removed.
After the resulting ingots were subjected to soaking at 1,150.degree. C.
for 30 min., they were hot rolled into a thickness of 3 mm at a finishing
temperature of 1,100.degree. C. Scale which was produced on the surface
owing to the hot rolling was removed by pickling them in a mixture
solution of nitric acid and hydrofluoric acid with a temperature of
66.degree. C. maintained. Thereafter, they were cold rolled into a
thickness of 1 mm, annealed at a temperature of 1,100 to 1,150.degree. C.
for 5 min. and cooled in water. Likewise, the scale produced on the
surface due to annealing was removed.
TABLE 1
______________________________________
Chemical Compositions in the Present and Reference Alloys
Unit: wt %
Alloy
No. C Ni Cr
Mo
Si
Mn
Others
______________________________________
1 0.02 11.62 20.56
6.75 0.97 -- 0.29
2 0.03 7.65 20.82
6.94
0.95
-- 0.28
3 0.02 6.60 21.96
6.59
1.14
-- 0.29
4 0.02 5.03 20.92
6.84
0.99
-- 0.28
5 0.02 4.27 21.36
6.52
1.09
-- 0.27
6 0.03 2.15 20.61
6.83
0.96
-- 0.26
7 0.02 9.11 21.66
6.90
0.76
-- 0.32
8 0.01 8.12 21.80
6.76
0.79
-- 0.29
9 0.01 6.05 21.96
6.55
0.69
-- 0.28
10 0.15 7.68 21.91
6.47
0.86
-- 0.29
11 0.15 6.81 21.88
6.41
0.93
-- 0.29
12 0.16 5.81 21.89
6.55
0.88
-- 0.32
13 0.02 7.17 23.33
6.85
0.51
0.32
0.35
14 0.03 5.25 23.63
2.84
0.52
0.38
0.37
15 0.12 7.28 23.43
6.80
0.59
1.06
0.32
Ti 0.25
16 0.04 7.40 23.54
6.83
0.56
1.13
0.39
Cu 0.84
17 0.13 7.36 23.61
6.75
0.57
1.12
0.33
18 0.09 5.52 21.15
6.01
0.72
1.02
0.35
19 0.02 6.34 21.12
5.95
0.61
1.01
0.35
20 0.10 2.21 22.31
6.14
1.12
1.03
0.34
21 0.09 11.12 20.93
6.05
1.34
0.51
0.33
22 0.12 6.53 20.27
5.69
1.26
0.56
0.32
23 0.15 6.23 21.92
5.52
1.26
0.65
0.25
Ti 0.48
24 0.16 6.59 21.40
5.61
1.34
0.65
0.25
Ti 0.43
25 0.03 4.01 21.36
6.52
1.21
0.56
0.29
26 0.02 3.99 21.42
6.30
1.25
0.70
0.31
27 0.03 4.19 21.45
6.27
1.21
0.64
0.28
28 0.02 6.05 28.01
7.03
1.01
-- 0.48
29 0.02 8.13 29.98
7.01
1.03
-- 0.47
30 0.02 10.08 29.45
7.12
1.06
-- 0.45
AISI304 0.07
8.61 19.59
0.74
0.61
-- 0.04
AISI316 0.08
11.06 16.97
2.57
0.52
-- 0.03
AISI316L
0.02
11.05 16.97
2.57
0.52
-- 0.03
SUS M329
0.02
7.75 21.66
-- 0.43 0.89
0.007
SUS329J1
0.06
5.68 23.05
1.34
0.34
0.47
--
SAF2507 0.03
7.00 25.00
4.00
0.80
1.2
0.30
UR52N+4 0.03 8.00 25.00
3.80
1.00
1.0
0.26
Cu 1.5
ZERON 100
0.03 9.00 26.00
4.00
1.00
1.0
0.30
W 1.0
Cu 1.0
______________________________________
EXAMPLE II
Stress Corrosion Cracking Test
Specimen Nos. 1 through 12 obtained in Example 1 were tested for stress
corrosion cracking. This test was carried out by a teach of constant
extension rate test (CERT) according to ASTM G 36-75. For test conditions,
cross-head speed was 4.41.times.10.sup.-6 cm/sec and initial deformation
rate was 1.35.times.10.sup.-5 /sec. The specimens were polished with SiC
abrasive paper Nos. 120 to 600, degreased with acetone, washed with
distilled water and then, dried. Final abrasion direction was rendered
parallel to the rolling direction.
For measuring fracture time, Specimen Nos. 1 to 12 were immersed in
respective 1L corrosion cells containing 42% MgCl.sub.2 with a temperature
of 154.degree. C. maintained. As a reference, AISI 304 alloy, commercially
available from Sammi Special Steel Co. Ltd, Korea, was used.
FIG. 1 shows the results of this stress corrosion cracking test for
Specimen Nos. 1 to 6 and FIGS. 2A and 2B show the results for Specimen
Nos. 7 to 12 and the reference, AISI 304 alloy. From these drawings, it is
revealed that the alloys according to the present invention are quite
superior to the reference in resistance to stress corrosion cracking.
EXAMPLE III
Pitting Test (Weight Loss Test)
Specimen Nos. 1 through 6 were subjected to a weight loss test according to
ASTM G 48. Following immersion of Specimen Nos. 1 to 6 in respective 10 wt
% FeCl.sub.3.6H.sub.2 O solutions for 24 hours, their corrosion rates were
evaluated by weight loss. As references, AISI 316L and SUS M329, both
commercially available from Sammi Special Steel Co. Ltd., Korea, were
used.
With reference to FIG. 3, there are shown the corrosion rates of the
specimens with the references. As apparent from this figure, Specimen Nos.
1 to 6 are stainless steels that are even more corrosion resistant than
AISI 316L alloy, and show superior corrosion resistance relative to SUS
M329, a duplex phase stainless steel.
EXAMPLE IV
Pitting Test
(Anodic polarization test in a test solution of 0.5N HCl+1N NaCl)
Specimen Nos. 1 through 6, 19, 20 and 22 to 27 were immersed in mixture
solutions of 0.5N HCl and 1N NaCl at 50.degree. C. Using a potentiostat,
potential was scanned from corrosion potential in the anodic direction to
obtain voltage-current curves. As reference alloys, AISI 316L and SUS
M329, both stainless steels commercially available from Sammi Special
Steel Co. Ltd., Korea, were used. The results are given as shown in Table
2 below.
From FIG. 4, it is recognized that all present alloys but No. 6 show wide
passive regions. This figure also shows that, in contrast with the present
alloys, the references, AISI 316L and SUS M329, show serious pitting,
which demonstrates rapid corrosion as the potential is increased. An
observation of the surfaces of Specimen Nos. 1 to 5 after the test
revealed that there was no pits on the alloy surface. Further, the present
alloys exhibit corrosion resistance comparable with that of titanium, an
expensive metal.
TABLE 2
______________________________________
Ferrite Passive
Passive
Alloy
Equi. Content
I.sub.crit
Region
Current
Pitt-
No. Cr/Ni % mVA/cm.sup.2
uA/cm.sup.2
ing
______________________________________
1 23.78/22.01
21 1300 1000.ltoreq.
150 X
2 24.20/18/08
33 1125
1000.ltoreq.
X
3 25.27/16.66
45 680
1000.ltoreq.
X
4 24.26/15.16
54 620
1000.ltoreq.
X
5 24.53/14.14
75 870
1000.ltoreq.
X
6 23.89/12.06
84 5700
350
O
19 23.00/18.80
50 673
1000.ltoreq.
x
20 25.14/16.82
80 742
490
OO
22 22.86/20.98
41 660
1000.ltoreq.
X
23 24.34/19.81
85 1031
800
O
24 24.03/20.47
79 1120
800
O
25 24.71/14.98
65 720
1000.ltoreq.
X
26 24.61/15.25
51 640
1000.ltoreq.
X
27 24.58/14.94
47 589
1000.ltoreq.
X
28 31.57/21.38
43 1090
1000.ltoreq.
X
29 33.55/23.20
49 850
1000.ltoreq.
X.5
30 33.17/24.63
61 1200
1000.ltoreq.
X
AISI 15.33/14/68
0 6100
170
OO --
316L
SUS 17.32/11.57
80 4500
--
-- OO
M329
______________________________________
note: X: none of pitting, OO: serious pitting
EXAMPLE V
Pitting Test
(Anodic polarization test in an artificial sea water test solution
according to ASTM D-1141-52)
Artificial sea water was prepared according to ASTM D-1142-52, to be used
for a test solution for Specimen Nos. 25 to 27 obtained in Example I. As
references, AISI 304 and AISI 316, both commercially available stainless
steels from Sammi Special Steel Co. Ltd., Korea, were used. Results were
given as shown in Table 3 below.
TABLE 3
______________________________________
Pitting Resistance in Artificial Sea Water Solution
according to ASTM D-1141-52
Passive
Passive
Current
Alloy Equi.
Region
Density
No. Cr/Ni
mV
uA/cm.sup.2
Pitting
______________________________________
25 24.71/14.98
1000.ltoreq.
<10 X
26 24.61/15.25
1000.ltoreq.
<10
X
27 24.56/14.00
1000.ltoreq.
<10
X
AISI 304 16.26/14.00
500 <10
OO
AISI 316 15.33/16.49
600 <10
OO
______________________________________
note: X: none of pitting, OO: serious pitting
EXAMPLE VI
The chromium/nickel equivalents of Specimen Nos. 13 to 17 obtained in
Example I were 25.96/19.28, 22.26/18.21, 26.13/21.98, 26.22/21.56, and
26.23/22.65, respectively. An anodic polarization test was carried out in
a mixture solution of 0.5N HCl and 1N NaCl, in the same manner as in
Example IV, so as to obtain data for corrosion resistance. The results of
testing Specimen Nos. 13 to 17 and SUS 329J1, a commercially available
duplex phase stainless steel, for mechanical properties and corrosion
resistance are given as shown in Table 4 below.
TABLE 4
______________________________________
Properties of Present and Reference Alloys
Passivity
Yield Tens. Current
Alloy
Str. Str. Elong.
I.sub.crit
Region
Density
No. kg/mm.sup.2
kg/mm.sup.2
% uA/cm.sup.2
mV .mu.A/cm.sup.2
Pitting
______________________________________
13 73.8 101.5 25.3 295 1010 11.2 X
14 73.2 98.9 29.2 3990
380
45.5
O
15 75.1 106.5 22.9 205
1010
24.2
X
16 76.3 109.2 28.4 150
1010
25.2
X
17 112.8 27.2 145
1010
9.6
X
SUS 68.1 81.2 23.5 8900
290
95.5
OO
329J1
______________________________________
Note: X: none of pitting; O: pitting; OO: serious pitting
As apparent from Table 4, the present alloys are quite superior to the
commercial available stainless steels in the mechanical properties and
corrosion resistance to the solution containing chloride ions.
EXAMPLE VII
Aging Heat Treatment
Using Specimen Nos. 13 and 15 obtained in Example I, an effect of aging
heat treatment was evaluated. The specimens were thermally treated at
temperatures ranging from 700 to 950.degree. C. in a mixture salt bath of
BaCl.sub.2 and NaCl. A series of tests, e.g. measurement of ferrite
content, intergranular corrosion test (according to ASTM 262 practice C),
pitting test (anodic polarization test in a solution of 0.5N HCl+1N NaCl
at 50.degree. C.) and mechanical test, were carried out for the
heat-treated specimens. The results are given as shown in Table 5 below.
Through point count method from optical micrographs of the specimens, the
ferrite contents of the specimens were obtained, showing about 15% at
850.degree. C. and 900.degree. C., smaller content than at any other
temperature. It was revealed that the ferrite content was not largely
affected by aging time (from 10 minutes to 3 hours).
The results of intergranular corrosion test say that the specimens both are
corroded at the highest rate at 700.degree. C. and at more reduced rate at
higher temperatures. Reduction of the corrosion rate as temperature is
increased is believed to be attributed to a fact that chromium in the
matrix structure is readily rediffused into sensitization region at high
temperatures.
From an observation of the surfaces of the specimens before and after the
anodic polarization test, it was revealed that initiation of pitting took
place at ferrite phase and its propagation does not have any preference
for ferrite and austenite phases.
EXAMPLE VIII
Effect of Aging Heat Treatment
Specimen No. 18 obtained in Example I was subjected to aging heat treatment
in a mixture salt bath of CaCl.sub.2 and NaCl at each temperatures of 550,
650, 750, 850 and 950.degree. C. for a period of 10, 30, 60 and 180
minutes. For this specimen, an observation of structure, a measurement of
ferrite content and an intergranular corrosion test according to ASTM A262
PRACTICE C were performed. With respect to intergranular corrosion rate,
an immersion test was carried out according to ASTM G48, with the same
anodic polarization test as in Example IV followed at 50.degree. C. in a
mixture solution of 0.5N HCl and 1N NaCl. The results are given as shown
in Table 6 below.
EXAMPLE IX
Effect of Aging Heat Treatment
Specimen Nos. 19, 20 and 22 to 24 obtained in Example I were subjected to
aging heat treatment. This treatment was carried out in a mixture salt
bath of CaCl.sub.2 and NaCl at each temperatures of 550, 650, 750, 850 and
950.degree. C. for a period of 10, 30 and 180 minutes. Likewise, there
were observations of structure, measurements of ferrite content and
intergranular corrosion tests. Further, pitting tests and mechanical tests
were carried out. The results are given as shown in Tables 5 and 6.
TABLE 5
______________________________________
Effect of Aging Heat Treatment
.sup.2 Aging Heat Treatment
.sup.1 Ferrite
Intergranular
.sup.3 Pitting
Alloy Content Temp.
Corrosion Rate
Potential
No. % .degree. C.
mg/m.sup.2 hr
mV(SHE)
______________________________________
13 35 700 4,250 no pitting
320 750
no pitting
800
290
870
250 850
no pitting
112 900
no pitting
15 40 3,043 700
no pitting
152
789
800
146
no pitting
144 850
no pitting
110 950
no pitting
22 41 1,200 550
no pitting
1,899
879
750
3,100
650
670 850
no pitting
125 900
no pitting
23 85 765 550
380
812
376
750
987
350
234 850
378
113 950
390
24 79 798 550
346
805
312
750
1,012
298
351 850
364
120 950
387
______________________________________
TABLE 6
______________________________________
Effect of Aging Heat Treatment
.sup.2 Aging Heat Treatment
Pit Passive
.sup.1 Ferrite
Intergranular
.sup.3 Pitting
Corr.
Current
Alloy
Content Temp. Corr. Rate
Potential
Rate Density
No. % .degree. C.
mg/m.sup.2 hr
mV (SHE)
mdd .mu. A/cm.sup.2
______________________________________
18 80 550 650 None 42 9
1,234
125
15
1,100 750
150
18
213 850
54ne
10
108 950
57ne
9
19 50 -- in anneal
None
-- 3
-- -- None
6
-- -- None
7
-- -- 842
6
-- -- None
10
-- -- None
5
20 80 -- in anneal
834
-- 5
-- -- 459
25
-- -- 478
18
-- 750
-- 513
13
-- 850
-- 543
11
-- 950
-- 650
8
______________________________________
note
.sup.1 when annealing treatment
.sup.2 treatment for 10 minutes
.sup.3 in anodic polarization test, none: no pitting generation
EXAMPLE X
Effect of Cold Working
With main instance of electrolytic iron, chromium, nickel, molybdenum,
Fe--Si, Fe--Cr--N, all commercially pure quality grade, 12 kg of alloy
Specimen No. 21 was prepared according to the composition as indicated in
Table 1, under a nitrogen atmosphere in a high frequency induction
furnace. At the moment parts containing pores were detected by radiography
were removed.
After the resulting ingot were subjected to soaking at 1,200.degree. C. for
30 min., it was hot rolled into a thickness of 3 mm. Scale which was
produced on the surface owing to the hot rolling was removed by pickling
it in a mixture solution of nitric acid and hydrofluoric acid with a
temperature of 66.degree. C. maintained.
Thereafter, it was thermally treated at 1,150.degree. C. for 10 min. and
then, quenched at room temperature to give a cold working rate of 0%, 10%,
30% and 60%, on the basis of thickness reduction. Following this, it was
subjected to recrystallization at 1,000.degree. C. for 5 min. The
equivalent value of Cr/Ni in the present alloy was 22.76/24.90.
An aging heat treatment was carried out in which the prepared specimen was
immersed in a mixture salt bath of CaCl.sub.2 and NaCl at each
temperatures of 650, 750, 850 and 950.degree. C. for a period of 10, 30
and 180 min. and cooled in water at room temperature.
An intergranular corrosion test (according to ASTM A262 PRACTICE C) and an
anodic polarization test (50.degree. C., 0.5N HCl+1N NaCl, scanning rate
20 mV/min) were performed. As for intergranular corrosion rate according
to aging temperature, it was the fastest at 750.degree. C., whereas the
slowest at 950.degree. C.
An X-ray diffraction analysis revealed that a sigma phase was detected in
the specimens aging-treated at 850.degree. C. or 950.degree. C. This sigma
phase was produced owing to the decomposition of ferrite upon aging heat
treatment and is believed to decrease a phase boundary, a priority place
of producing crystal nucleus of carbide, contributing to a reduction of
corrosion rate.
In case of performing both cold working and heat treatment, large working
rate brought about more reduction in grain size. As for corrosion rate
according to grain size, it was the largest for the largest grain size
which resulted from the heat treatment at a temperature of 650.degree. C.
or 750.degree. C. On the other hand, as the grain size becomes smaller,
the corrosion rate became reduced. This says that the degree of
sensitization increases with large coarse size.
Where aging heat treatment was not executed, in contrast, the corrosion
rate became increased with fine grain size resulting from
thermo-mechanical treatment in anodic polarization test. This is
attributed to a fact that the initiation point of pitting becomes
relatively abundant as the grain size is smaller. Such thermo mechanical
treatment specimens were subjected to aging heat treatment and then, to
anodic polarization test. Of the resulting specimens under conditions of
650.degree. C. and 30 min., one with the smallest grain size was of the
best anodic polarization resistance.
EXAMPLE XI
In this example, Specimen Nos. 2 through 5 were tested for the effect of
cold working. The annealed specimens of Example I were cold rolled in each
rates of 0, 10, 30, 40, 50 and 60%, followed by carrying out stress
corrosion cracking test (42% MgCl.sub.2, ASTM STANDARD G 36-75) and
mechanical test.
With respect to the effect of cold working on stress corrosion cracking
resistance, Specimen No. 2, which was rich in austenite, became high in
resistance as the cold working rate was more increased. On the other hand,
the other specimens, relatively rich in ferrite, became low in resistance
with increased cold working rate. This tendency is believed to be
attributed to a fact that the external stresses all are exhausted to work
harden the soft austenite and the austenite thus work-hardened prevents
movement of dislocation, thereby inhibiting the propagation of crack.
However, if ferrite is abundant, the external stresses cause an internal
deformation in the ferrite, which forces into the propagation of crack.
After Specimen No. 4 was cold worked, mechanical properties were measured.
Under the working rate of 0%, it showed a yield strength of 50
kg/mm.sup.2, a tensile strength of 75 kg/mm.sup.2 and a Vickers hardness
of 280. Under the working rate of 60%, these mechanical properties were
improved, e.g. a yield strength of 100 kg/mm.sup.2, a tensile strength of
120 kg/mm.sup.2 and a Cickers hardness of 395.
EXAMPLE XII
Making of Stainless Steel
With substance of electrolytic iron, chromium, nickel, molybdenum, Fe--Si,
Fe--Cr--N, all commercially pure grade, 30 kg of each of alloy specimens
was prepared according to the compositions as indicated in Table 7, in a
high frequency vacuum induction furnace.
After the resulting ingots were subjected to soaking at 1,250.degree. C.
for 120 min., they were hot rolled into a thickness of 4 mm. Scale which
was produced on the surface owing to the hot rolling was removed by
pickling them in a mixture solution of nitric acid and hydrofluoric acid
with a temperature of 66.degree. C. maintained. Thereafter, they were cold
rolled into a thickness of 1 mm, annealed at a temperature of
1,125.degree. C. for 5 min. and cooled in water. Likewise, the scale
produced on the surface due to annealing was removed.
TABLE 7
__________________________________________________________________________
Chemical Composition of the Present Alloys
Unit: wt %
Alloy
No.
C
Ni
Mo
Mn
N
Others
__________________________________________________________________________
31 0.04
7.90
23.20
5.70
0.60
0.03
0.33
Ti 0.65
32 0.03
5.50
25.70
4.30
0.60
0.02
0.36
33 0.03
5.60
26.30
5.00
0.60
0.02
0.43
34 0.03
5.20
21.00
6.80
1.00
1.90
0.27
Ti 1.5
W 2.5
35 0.04
5.10
22.30
4.60
1.00
1.90
0.27
Ti 1.4
W 2.6
36 0.04
3.80
24.80
4.10
1.00
3.10
0.35
Ti 1.7
W 2.6
37 0.02
7.10
19.90
6.60
0.90
0.06
0.21
Ti 0.71
38 0.03
7.00
23.00
5.60
0.5O
0.05
0.33
B 0.001
Ti 0.72
Al 0.001
39 0.03
7.00
26.00
5.10
0.50
0.50
0.41
B 0.001
Ti 0.72
W 0.7
40 0.03
4.58
30.55
2.50
0.50
0.5O
0.51
B 0.005
Ti 0.75
Al 0.012
41 0.03
7.90
33.70
3.10
0.80
0.60
0.44
B 0.001
Ca 0.005
42 0.03
8.20
34.90
2.50
0.60
0.50
0.49
B 0.001
Ca 0.002
V 0.5
Mg 0.003
43 0.03
6.20
20.50
5.40
0.61
0.41
0.26
Cu 1.9
44 0.02
7.40
23.50
4.30
0.42
0.53
0.34
Cu 0.72
45 0.03
8.50
25.90
5.00
0.53
0.56
0.36
Cu 0.65
46 0.03
7.50
23.10
5.60
0.61
0.64
0.32
Cu 0.71
W 1.2
47 0.03
7.00
23.30
5.50
0.50
0.62
0.33
Cu 0.85
Ti 0.75
__________________________________________________________________________
When compared with specimens obtained in Example I,
Specimen Nos. 38 through 42 each which contains boron, aluminum, calcium,
magnesium or combinations thereof shows improved hot workability. That is
to say, there was a remarkable reduction in edge crack that was used to
appearing at the opposite edges of hot plate.
EXAMPLE XIII
Comparison of Corrosion Resistance
Specimen Nos. 31 and 37 obtained in Example XII were immersed in a 6%
FeCl.sub.3 solution and separately, a mixture solution of 7% H.sub.2
SO.sub.4, 3% HCl, 1% FeCl.sub.3 and 1% CuCl.sub.2, in order to measure
their critical pitting temperatures. For this, corrosion rates were
calculated from measurements of the weight loss after immersing them in
the solutions for 24 hours at a temperature interval of 50.degree. C. The
results are given as shown in Table 8 below.
For measurement of anodic polarization resistance, the specimens were
immersed in a mixture solution of 0.5N HCl and 1N NaCl at a temperature of
70.degree. C. maintained and separately, in a 22% NaCl solution at a
temperature of 80.degree. C. maintained. Using a potentiostat, potential
was scanned from the corrosion potential in the anodic direction to obtain
voltage-current curves. As a reference, SAF2507, a commercially available
stainless steel, were used. The Results are given as shown in Table 8
below. FIGS. 5 and 6 show the superior corrosion resistance of the present
alloys.
TABLE 8
______________________________________
Critical Pitting Temperature and Anodia Polarization Resistance
Critical Pitting Temp. .degree. C.
Anodic Polarization Resist.
Alloy
6% .sup.1 Mixed
70.degree. C.
80.degree. C.
No. FeCl.sub.3
Solution
0.5N HCl + 1N NaCl
22% NaCl
______________________________________
31 .gtoreq.bp.
95-90 no pitting
37 95-90
85-80 no pitting
SAF2507
85-80 65-60 serious pitting
______________________________________
.sup.1 7% H.sub.2 SO.sub.4 + 3% HCl + 1% FeCl.sub.3 + 1% CuCl.sub.2
EXAMPLE XIV
Effect of Aging Heat Treatment
In order to evaluate the effect of titanium on aging heat treatment,
Specimen Nos. 31 to 33 and 37 were subjected to aging heat treatment at
800.degree. C. for 1 hour and then, to intergranular corrosion test (Huey
Test). Corrosion rates of the specimens were 131, 667, 635 and 159
mg/m.sup.2 hr, respectively.
It was revealed that Specimen No. 31 which contained an appropriate amount
of titanium was superior to Specimen Nos. 32 and 33, devoid of titanium,
in corrosion resistance even after aging heat treatment. FIGS. 7 and 8
show the corrosion resistance of the present alloys and a reference after
heat treatment.
EXAMPLE XV
Specimen Nos. 37 and 43 through 47 obtained in Example XII were immersed in
10% sulfuric acid solution at 80.degree. C. for 24 hours and separately,
in 10% hydrochloric acid solution at 25.degree. C. for 24 hours, to
measure corrosion rates thereof. The results are given as shown in Table 9
below. As apparent from Table 9, addition of copper allows the alloy to be
improved in corrosion resistance to acid.
TABLE 9
______________________________________
Effect of Cu Addition
Corrosion Rate Corrosion Rate
Alloy No.
(80.degree. C., 10% H.sub.2 SO.sub.4, mdd)
(25.degree. C., 10% HCl, mdd)
______________________________________
37
139 959
43 71 932
44 56 899
45 55 901
46 47 786
47 49 790
SAF 2507
84 3,362
UR52N+ 115
2,004
Zeron 100
403 2,546
______________________________________
Other features, advantages and embodiments of the present invention
disclosed herein will be readily apparent to those exercising ordinary
skill after reading the foregoing disclosures. In this regard, while
specific embodiments of the invention have been described in considerable
detail, variations and modifications of these embodiments can be effected
without departing from the spirit and scope of the invention as described
and claimed.
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